Optical techniques based on the Near-infrared spectral window have made significant progress in biomedical research in recent years. The relative low absorption and low scattering in the 600-1000 nm spectral range allow detection of photons that have traveled through several centimeters of biological tissue. Coupled with accurate models of light propagation, NIR techniques enable imaging of deep tissue with boundary measurements using non-ionizing, low dose radiation.
The interest in NIR techniques is fueled by the ability of the techniques to monitor functional tissue parameter such as oxy- and deoxy-hemoglobin and the development of appropriate low cost instrumentation. Based on these qualities, NIR optical imaging is expected to play a key role in breast cancer detection, characterization and monitoring through therapy; brain functional imaging and stroke monitoring; muscle physiological and peripheral vascular disease imaging. For all these applications, NIR techniques rely on endogenous contrast such as tissue hemodynamics.
One particular example of a potential application of optical imaging is breast cancer. Breast cancer is a major health problem worldwide. In North America, it is estimated that in the United States approximately 266,471 (American Cancer Society. Cancer Facts and Figures 2004. Atlanta, Ga., 2004) and in Canada 21,200 (Canadian Breast Cancer Foundation, Breast Cancer Facts, Toronto, ON, 2004) new cases of breast cancer will be diagnosed among women in 2004. Furthermore, 40,110 women in the U.S and 5,200 women in Canada will die from it. Incidence rates have begun to stabilize over the last ten years but continue to increase. It is estimated that one in eight American women and one in nine Canadian women will develop breast cancer at some point during their lifetime. But thanks to earlier detection and more effective treatments, the mortality rate for women of all races combined declined by 2.3% annually between 1990 and 2000.
Optical techniques for imaging the breast can be tracked back to the late 20's with the work of Ewings (Ewing, 3rd edition. Philadelphia, Saunders, Philadelphia, 1928) and Cutler (Cutler. Surg. Gynecol. Obstret. (1929); 48:721-9) who presented the first clinical results using optical techniques. Since then the technology has evolved, leading to enhanced systems in the 70's (Gros et al. J Radiol Electrol Med Nucl, (1978); 53:297-306) and commercial products in the 80's (Carlson. Spectrascan, S. Windosr, Conn., 1982) (10 Litescan, Spectrascan) that ultimately failed to receive acceptance as clinical modalities due to inconclusive results (Alveryd et al. Cancer (1990); 65:1671-1677).
Despite this setback, optical techniques have received steady attention in the last decade (Kincade. Laser Focus World January 2004; 130-4). The main reasons for this surge in interest reside in the development of new mathematical models able to describe accurately and quantitatively the propagation of light in biological tissues (Yodh et al. Physics Today (1995); 48:34-40). These mathematical models applied to multi-spectral measurements, are the foundation of diffuse optical spectroscopy (DOS) and diffuse optical tomography (DOT) (X Intes et al. Radiologic Clinics Of North America, January 2005).
Alternative methods for breast cancer detection such as X-ray mammography are widely used but do not always provide enough information to enable a conclusive diagnosis to be made. Thus other, complementary tests must be used, such as biopsy or blood tests, which can be invasive and may require a long time to complete.
There is therefore a need for methods to better detect spatial variations of chromophores in biological tissue, and to image spatial distributions thereof.